Concrete used to be the Roman Empire’s construction material of choice and was used in monuments such as the Pantheon in Rome as well as in wharves, breakwaters and other harbor structures. No wonder concrete structures are meant to last a lifetime. Usually concrete structures have a design life of 50 or 100 years, but unfortunately many of today’s structures are not living up to expectations. It is not an overstatement to say that water causes almost everything bad that happens to concrete. Water carries chemical contaminants into the concrete, which can corrode the steel reinforcement, contribute to expansive and erode the surface. You will want to do everything you can to stop the premature deterioration of concrete infrastructure and to enhance the durability of your concrete and here are five suggestions:
Durability starts with a concrete mix design suitable for the service conditions that the concrete will be exposed. The amount of water specified may be the single most important component of the mix design. A low water-cement ratio between 0.40 and 0.50 is normally recommended for concrete frequently subjected to severe exposure. Do not add extra water on site. Additional water will lower strength and increase permeability and the likelihood of scaling.
No single mix design can be durable in every situation. For example, the mix design should consider using sulphate resistant cement, when concrete is exposed to sulphate contaminated soil. However this mix design is not appropriate when a concrete structure is exposed to the cycles of freezing and thawing in a wet environment. In this case, the mix design should include an air-entraining admixture since entrained air will help to protect the concrete under these conditions.
Water-reducing admixtures reduce the amount of water required for a given consistency and contribute to the degree of cement hydration and, in turn, produce a more durable cement paste. Increased hydration results in more durable concrete.
Other important components of the mix design are the aggregates. Contaminated aggregates can cause pop-outs or alkali silica reactivity (ASR), and off-spec cement can cause finishing problems with inconsistent setting times.
Construction joints are a common weak point in the structure where water will often find its way in and through it. Corrosion of the reinforcing steel from water penetrating through the joints can cause damage to your structure. Be sure to design your structure with adequate, pre-determined control joint locations in order to avoid random cracking and make sure that each joint employs a reliable waterproofing system of its own. The best long-term protection at joint locations is due to a joint waterproofing system that includes crystalline technology, which self-seals random cracking and thus provides added reliability and protection.
Prevent Premature Moisture Loss During Placing
Keeping the concrete moist and at an adequate temperature helps to provide proper hydration of the cement for maximum strength, increased density, lower permeability, and greater resistance to scaling. For durable concrete proper placing and finishing are essential as excessive or premature finishing contributes to blistering and reduces surface air content and durability. The popular practice of “blessing” concrete with water during troweling, increases the probability of surface scaling and spalling. The increase in the water-cement ratio at the surface weakens the concrete at the near-surface wear zone. The use of an evaporation retardant in rapid moisture loss conditions can eliminate the need for “blessing.” This will prevent surface crusting and plastic shrinkage cracking and reduce overall placement costs while increasing placement crew productivity and overall concrete quality.
You may have to adjust the mix designs to match the conditions, but there is one factor that is common to all good concrete and that is low permeability. For lower permeability good mix proportioning by reducing the amount of free water in your mix and using water reducing admixtures or plasticizers is essential. A waterproofing admixture should be used to protect concrete from water intrusion.
Curing means maintaining conditions that will allow the concrete to harden and gain strength optimally and this step is central to extending the life of your concrete. Proper temperature and humidity are most important. The temperature should be above 10°C for strength development to proceed at a reasonable pace. Moisture must be maintained, especially in the first few days in order to promote cement hydration and keep the concrete from drying, shrinking and cracking.
Protect Concrete From De-icing Salts After Curing
Frequent wetting and drying during the early age of concrete will cause crazing, scaling, lower concrete strength, and increase the possibility of scaling. It is essential that concrete be protected from freezing temperatures during the first days of hydration; when its low strength and high moisture content make it most susceptible to freeze/thaw damage. Where the presence of deicer agents cannot be eliminated, treat the surface, with something like Aquapel, to prevent the migration of nearly 90 percent of deicer salts and effectively repel a high percentage of the moisture that will cause deterioration.
Concrete must be placed properly and consolidated into a solid mass that is free from any voids or rock pockets. Voids most commonly occur below or behind reinforcing steel bars – especially in areas where there are many bars together. Be sure to properly vibrate all concrete with special attention around congested reinforcement. Rock pockets are areas where there is not enough
cement paste and fine aggregate to close the space between large aggregate. Make sure there is no standing water prior to concrete placement and do not move concrete large distance from where it is initially poured. The use of self-consolidating concrete can alleviate many of these concerns.
With proper mix design, permeability reduction, and attention to good jointing, placing and curing practices, one can create durable concrete structures that will resist the development of corrosive conditions and other deteriorating mechanisms.
Davis Concrete is a family-owned and operated concrete business that provides Florida homeowners and contractors with the very best building materials in Clearwater, Tampa, Bradenton and Lakeland.